All marine creatures have evolved in the sea, which is a very stable environment with massive water buffering capacity.
Buffering capacity means that because of the oceans vast size, it has stable
⪼ specific gravity
⪼ other chemicals
This is unlike freshwater fish whose environment can often be affected dramatically by events like landslides, snowmelt, drought and flooding causing massive changes in water conditions.
This lack of chemical and physical change in seawater means marine fish and invertebrates do not possess the physiological ability to adapt to different water conditions that freshwater species possess.
For us as marine aquarists this means to ensure our marine pets survive and thrive we need to provide optimal water quality at all times. This is especially true for reef aquariums where corals have a much more specific set of requirements than marine fish do.
In my experience poor water quality is the number one reason things go wrong with marine life in peoples saltwater aquariums. It is the leading cause of death resulting from stress and shock of chemical and physical fluctuations in the water.
So the key here is good quality, stable saltwater which is actually easy to achieve.
The first main consideration in top quality water is a really good purified water source.
Water from the tap should never be used as it contains high levels of phosphate, nitrate and heavy metals among other chemicals you do not want in your aquarium.
The best way to obtain purified water is to purchase a reverse osmosis water filter for tap water; this investment will soon pay itself back to you. Alternatively you can purchase pre-filtered / distilled water or pre-prepared saltwater.
Next up would be a high quality salt mix if you are using filtered freshwater, this should closely replicate the chemical composition of natural seawater (NSW) and will add in all the vital minerals and trace elements your marine life need for health.
Now that we have the nuts and bolts of a good quality saltwater source figured out the next major parameter required for high quality saltwater aquarium water are a high level of oxygenation and strong water movement.
Oxygenation can come from a good protein skimmer, but also airstones and water pumps and powerheads; any thing that moves water around rapidly or puts bubbles into it.
All marine life breath oxygen so this is important especially as saltwater absorbs 250 times less oxygen than freshwater. So you want a high water turnover at the surface of the tank where oxygen can be absorbed.
Strong, multidirectional water movement is especially important for corals and other sessile invertebrates; to bring them their plankton food and nutrients, clean them of detritus and oxygenate them.
You can never really have too much water movement in a marine aquarium; it stirs up detritus and prevents anoxic zones forming where organic debris could start decaying anaerobically releasing deadly toxins into the water.
A series of powerheads pointed at each other will do the job; you can even put them on timers to emulate ebb and flow. You can also get advanced programmable propeller pumps to simulate different reef water movements and waves.
The ideal water temperature for a saltwater aquarium is 77-80°F (25-27°C) this will be achieved using 2 heaters depending on your aquarium size. If you live in a warm house/climate or have metal halide lights you may need to purchase a water chiller too, especially if you have a reef aquarium. A chiller is particularly useful if your temperatures are fluctuating more than 4 degrees (F) at a time that will stress out your marine life.
To accurately keep the temperature in the range 77-80°° (25-27C) you will need an accurate thermometer. Even better is a temperature controller unit hooked up to your heater that will keep temperature fluctuations to a minimum and lead to less stressed marine life.
Good quality marine aquarium water will have an ideal pH (alkalinity) of 8.3. To keep the pH at this level you will need to test it regularly using a pH test kit. In a fish only set up the pH can range from 7.6 to 8.4 without causing harm to the fish. In a reef tank the invertebrates are more sensitive requiring a pH range of 8.0 to 8.4.
In a saltwater aquarium set up the pH is normally likely to go down overall (become more acidic) mostly from organic acids produced by biological waste.
Usually the waters buffering system can retard this pH drop to a degree but it does begin to wear out after a while and the buffering chemicals such as calcium, carbonate, and bicarbonate need replenishing. This is where the term “alkalinity” comes in; it is the waters buffering ability to stay alkaline in the presence of these acids.
The best way to stabilize pH is with regular partial water changes, which replenishes the aquariums buffering capacity, and also adds back vital trace elements that get used up by marine life.
If the pH is too low you can add a pH reduction product. If its too high you can add a pH increaser product or baking soda, you just have to be careful as the basic baking soda can burn marine life before it gets diluted.
Carbonate hardness (dKH) is the measure of alkalinity or buffering capacity of saltwater, which is essentially the pH stabilizer. Ideally you will have 9-12dKH this will provide good buffering against pH fluctuations. This can best tested for with carbonate hardness test kits.
Calcium Reactors are high tech pieces of equipment that offer a fantastic solution to the problems of pH fluctuations, carbonate hardness and calcium dosing (especially important for reef aquariums) in one unit. Aquarium water, CO2 gas and calcium carbonate are combined in a reaction chamber to produce injections of calcium bicarbonate that provides the calcium vital for invertebrates’ growth, adequately buffer aquarium water and corrects pH. Calcium reactors are strongly recommended for reef aquariums.
Specific gravity is a weight ratio of one liter of a saltwater compared to one liter of water and is temperature dependent. It is essentially a way to measure the salt content in your aquarium and to make sure it is as close as possible to seawater. This is an important parameter of good quality aquarium water.
Hydrometers are used to measure specific gravity that ideally will be 1.023. Conductivity however is a more accurate measure. The conductivity of your aquarium water should be 50.1 ms/cm @ 25°C this can be measured using conductivity probe.
Specific gravity will change with the event of evaporation, which leaves behind salt and increases specific gravity and salinity that will harm marine life; this is why we need to measure it regularly. If its too low add more salt mix, too high add more purified water.
Its important to note that marine life from the Red Sea which has a higher salt content than the rest of the worlds oceans need their specific gravity at 1.024-1.025 to be optimally healthy.
Ammonia/Ammonium, Nitrite and Nitrate are all Nitrogenous compounds form the breakdown of biological waste and organic matter. All should be scrubbed up by biological filtration and be kept to an absolute minimum.
Test kits are readily available for each compound. Ammonia and Nitrite are particularly toxic to marine life and should only be present during biological cycling in a new aquarium, ideally you will have a zero reading for these.
Nitrate is the end product of nitrification and again we want as little of this as possible Fish only aquariums should have less than 50ppm (~20mg/liter). Reef and FOWLR (fish only with live rock) aquariums should have less than 20ppm (~3mg/liter).
All the major water quality and water chemistry components have now been covered for fish only aquariums if you have a FOWLR aquarium with a few corals and other invertebrates or a reef aquarium you must also consider the following parameters:
Phosphate levels must be kept to a minimum, even though phosphate is a major nutrient for corals. Ideal phosphate levels should be less than 0.05ppm (or 0.01 mg/liter).
Phosphate can be introduced to the aquarium in many forms (unfiltered water, overfeeding, non-phosphate free salt mixes or medias) and excess leads to algae blooms that are particularly undesirable in reef aquariums where it grows on the corals and can suffocate them.
You should test new aquarium products for phosphate by putting them in water for an hour then testing that water. Phosphate test kits are cheap and easy to use. Phosphate removal media is a very good way to get rid of excess phosphate from your system. Regular water changes should help you keep on top of phosphate levels.
Calcium is a very important compound in reef aquariums as hard corals, molluscs, soft corals and crustaceans use a lot of this compound to build their skeletons. Calcium is used up from the water fast in an established aquarium; as much as 15mg/liter per day!
Calcium levels are ideally 420mg/liter, which is the same as NSW. Test kits can be used to give a handle on this vital element calcium can be dosed into your aquarium using a calcium reactor and not simply by adding coral sand to your aquarium as many people incorrectly think. Depletion of calcium also reduces the buffering capacity of marine aquarium water too.
Other important elements for thriving invertebrates in reef aquariums are Iodine and Iron, which are both beneficial for fish too commercial additives and test kits are easily available. The ideal levels for each are 0.5 mg/liter for iodine and 0.05 mg/liter for Iron. Both are used up from the water by marine life so will need to be tested for regularly.
Stony reef building corals also need Strontium, ideally at 8 mg/liter for building up their skeletons. Strontium is quickly depleted from the water so will need to be added regularly.